A retrospective quantitative CT study reveals subclinical pulmonary vascular loss and right middle lobe vulnerability in PM/DM patients without interstitial lung disease

Background Connective tissue disease-associated interstitial lung disease (CTD-ILD) is triggered by pulmonary vascular injury. However, early subclinical lung changes in polymyositis/dermatomyositis (PM/DM) patients without ILD (PM/DM-non-ILD) remain undetectable by conventional imaging. Objective To evaluate microvascular integrity and parenchymal features in PM/DM-non-ILD patients using quantitative computed tomography (QCT). Additionally, this study aims to identify associations between QCT parameters and laboratory markers of inflammation and autoimmunity, and evaluate the performance of QCT metrics in detecting subclinical lung pathology. Method This retrospective study included 71 PM/DM-non-ILD patients and 71 age- and sex-matched healthy controls were enrolled. Chest high-resolution CT (HRCT) images, laboratory data, and clinical records were collected. All HRCT scans were analyzed using the “Digital Lung” platform to obtain QCT parameters, including lung volume, density, and intrapulmonary vascular volumes (IPVV) for the whole lung and individual lobes. Standardized IPVV was calculated by dividing the vascular volume by the corresponding lobe volume to account for lung volume differences. QCT parameters and laboratory results were compared, their correlation analyzed, and diagnostic performance evaluated using receiver operating characteristic (ROC) analysis. Results In PM/DM-non-ILD patients, standardized IPVV was notably lower in the entire lung and each lobe compared to healthy controls ( P < 0.05). Hematological analysis of PM/DM-non-ILD patients showed a pattern consistent with immune dysregulation, characterized by significantly reduced levels of hemoglobin, lymphocytes, eosinophils, and basophils (including their percentages), alongside elevated monocytes, monocyte percentage, neutrophil percentage, neutrophil-to-lymphocyte ratio, and platelet-to-lymphocyte ratio compared to controls (all P < 0.05). QCT parameters showed significant correlations with key hematologic inflammatory indicators ( P < 0.05). Conclusion The decreased standardized intrapulmonary vascular volume (IPVV), across all lung lobes (whole lung, right lung, left lung, right upper lobe, right middle lobe, right lower lobe, left upper lobe, left lower lobe) indicates its potential as a sensitive marker for early pulmonary involvement in patients with polymyositis/dermatomyositis without interstitial lung disease. The distinct vascular changes in the right middle lung lobe may serve as a discriminative indicator for subclinical lung pathology in this population, potentially reflecting underlying immune dysregulation.